Federal investigators have questioned two brothers in connection with the death of a U.S. Border Patrol agent last month, according to a court document filed in New Mexico.

Rogelio Martinez suffered severe head injuries and broken bones Nov. 18 near Van Horn, Texas, and died in an El Paso hospital hours later. A second agent, whom the document identified for the first time as Stephen Garland, was severely injured and has said he doesn’t remember anything from the evening. His whereabouts are unknown.

Immediately following Martinez’s death, President Trump called the incident an attack and reiterated on Twitter that a border wall must be built. The National Border Patrol Council has insisted that the agents were attacked by undocumented immigrants or drug smugglers and has called for more resources to secure the border.

According to the court document, signed Dec. 2 and unsealed briefly over the weekend, FBI investigators questioned Antonio and Daniel Muñoz and found red-stained fabric in a 2004 Pontiac Grand Am that the brothers were traveling in.

Both Antonio and Daniel have denied any involvement in the death of Martinez. Neither has been arrested or charged in connection with the death. Antonio Muñoz is in Border Patrol custody for allegedly crossing the border illegally.

The FBI is seeking information into death of Border Patrol agent Rogelio Martinez via digital billboards that went up last week throughout the U.S. Southwest.

“The FBI is aggressively addressing all leads and tips,” FBI spokesperson Jeanette Harper said. “The latest lead is one of the many we are pursuing.”

Harper referred to an announcement by the FBI on Monday that it is doubling a reward -- from $25,000 to $50,000 -- for information that could lead to solving the case.

The agency also issued a new poster in Spanish, calling for information.

Separately, Texas is also offering a $20,000 reward for information.

“When you have that kind of money you’re going to get many leads, including very questionable ones,” said one of the people with direct knowledge of the investigation. “This appears to be the case here.”

According to the court document, an informant came to federal agents with a tip based on secondhand information that the brothers were present with a group of unknown undocumented immigrants during the alleged assault on the two agents with rocks.

A woman traveling with the brothers told agents last Friday that she overheard Antonio Muñoz speaking to someone about crossing into the United States with drugs “including methamphetamine and heroin,” according to the document.